Pixel readout architecture for full well capacity extension

ABSTRACT

Certain aspects relate to systems and techniques for full well capacity extension. For example, a storage capacitor included in the pixel readout architecture can enable multiple charge dumps from a pixel in the analog domain, extending the full well capacity of the pixel. Further, multiple reads can be integrated in the digital domain using a memory, for example DRAM, in communication with the pixel readout architecture. This also can effectively multiply a small pixel&#39;s full well capacity. In some examples, multiple reads in the digital domain can be used to reduce, eliminate, or compensate for kTC noise in the pixel readout architecture.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is related to U.S. patent application Ser. No.14/562,444, filed on Dec. 5, 2014, entitled “PIXEL READOUT ARCHITECTUREFOR FULL WELL CAPACITY EXTENSION,” the contents of which aresubstantially identical and hereby incorporated by reference herein.

TECHNICAL FIELD

The systems and methods disclosed herein are directed to photo sensors,and, more particularly, to full well capacity extension.

BACKGROUND

Full well capacity is the largest charge a pixel can hold beforesaturation which results in degradation of the signal. When the chargein a pixel exceeds the saturation level, the charge starts to filladjacent pixels, a process known as blooming. The sensor also starts todeviate from a linear response and hence compromises the quantitativeperformance of the camera.

The dynamic range of a camera is typically defined as the full wellcapacity divided by the camera noise and relates to the ability of acamera to record simultaneously very low light signals alongside brightsignals. Dynamic range, the ratio between the well capacity and noisefloor, is a measure of how well a sensor can measure an accurate signalat low light intensities all the way up until it reaches full wellcapacity. Given a similar noise floor, the dynamic range of a sensingelement is proportional to its well capacity, which is typicallyproportional to the size of such light sensing elements. Larger pixelshave greater full well capacity and accordingly higher dynamic range.

In digital imaging, the dynamic range of a complementarymetal-oxide-semiconductor (CMOS) sensor may, at times, be insufficientto accurately represent outdoor scenes. This may be especially true inthe more compact sensors which may be used in mobile devices, forexample in the camera on a mobile telephone. For example, a typicalsensor used in a mobile device camera may have a dynamic range ofapproximately 60-70 dB. However, a typical natural outdoor scene caneasily cover a contrast range of 100 dB between light areas and shadows.Because this dynamic range is greater than the dynamic range of atypical sensor used in a mobile device, detail may be lost in imagescaptured by mobile devices.

Further, the trend in digital cameras is toward smaller pixels to drivemegapixel count upward or to enable smaller sensor areas, and smallerpixels can suffer from more limited full well capacity than largerpixels. Particularly in mobile sensors, the trend in pixel size is toshrink the pixel size and also sensor area and attempt to maintainperformance through advanced processing. Reduced full well capacity,reduced quantum efficiency, and reduced photosensitivity of thesub-micron pixel detector array has greatly degraded signal-to-noiseratio (SNR) and dynamic range of the image sensor. In addition, highercross-talk of the reduced pixel size results in image quality issues forexample poor modulation transfer function (MTF) and color fidelity.

SUMMARY

The foregoing problems, among others, are addressed in some embodimentsby the pixel readout architecture and techniques for full well capacityextension described herein. An additional storage capacitor included inthe pixel readout architecture can enable multiple charge dumps from apixel in the analog domain, extending the full well capacity of thepixel. Further, multiple reads can be integrated in the digital domainusing a memory, for example DRAM, in communication with the pixelreadout architecture. This also can effectively multiply a small pixel'sfull well capacity. In some embodiments, multiple reads in the digitaldomain can be used to reduce, eliminate, or compensate for kTC noise inthe pixel readout architecture.

Accordingly, one aspect relates to an imaging system comprising a pixelarray including a plurality of pixels, each of the plurality of pixelscomprising a photosensor; a plurality of readout circuits for readinglight integrated in pixels coupled thereto, each of the plurality ofreadout circuits comprising a storage capacitor in communication with atleast one pixel of the plurality of pixels, the storage capacitor havingcapacitance for storage of an accumulated charge representing aplurality of charge dumps from the at least one pixel, each of theplurality of charge dumps comprising a charge representing the lightintegrated in the at least one pixel; an analog to digital converter incommunication with the storage capacitor to receive the accumulatedcharge from the storage capacitor and convert the accumulated chargeinto a digital signal; a memory configured to store the digital signal;and a noise compensation module configured to perform at least two readsof the pixel array to reduce effects of kTC noise of the storagecapacitor on captured image data.

Another aspect relates to a method for reducing an effect of kTC noiseon captured image data, the method comprising performing a first read ofa pixel array comprising a plurality of pixels by, for each readoutcircuit of a plurality of readout circuits coupled to the pixel array,resetting a photosensor of at least one pixel coupled to the readoutcircuit, resetting a storage capacitor coupled to the at least onepixel, performing a first read of the storage capacitor, the first readrepresenting a kTC noise of the storage capacitor, and reading out theaccumulated charge from the storage capacitor to a memory; performing asecond read of the pixel array by, for each readout circuit of theplurality of readout circuits, integrating light in the at least onepixel coupled to the readout circuit, accumulating, in the storagecapacitor, an accumulated charge representing a plurality of chargedumps from the at least one pixel, each of the plurality of charge dumpscomprising a charge representing the light integrated in the at leastone pixel, and reading out the accumulated charge from the storagecapacitor to the memory; and using the first read to reduce the effectof the kTC noise of the storage capacitor on the second read.

Another aspect relates to an imaging apparatus comprising photosensingmeans for collecting a charge representing light integrated in thephotosensing means; a plurality of readout means coupled to thephotosensing means, each of the plurality of readout means comprising astorage capacitor having capacitance for storage of an accumulatedcharge representing a plurality of charge dumps from a portion of thephotosensing means coupled thereto, each of the plurality of chargedumps comprising a charge representing the light integrated in theportion of the photosensing means; means for reading a kTC value of thestorage capacitor from the storage capacitor; and means for using thekTC value to reduce the effects of the kTC of the storage capacitor onthe accumulated charge.

Another aspect relates to a non-transitory computer-readable storagemedium storing instructions that, when executed, cause a processor toperform operations comprising receiving, from each readout circuit of aplurality of readout circuits coupled to a pixel array comprising aplurality of pixels, a kTC value representing kTC noise of a storagecapacitor coupled to at least one pixel of the plurality of pixels, thestorage capacitor having capacitance for storage of an accumulatedcharge representing a plurality of charge dumps from the at least onepixel, each of the plurality of charge dumps comprising a chargerepresenting the light integrated in the at least one pixel; receiving,from each of the plurality of readout circuits, the accumulated chargevalue accumulated in the storage capacitor; and using the kTC value toreduce an effect of the kTC noise of the storage capacitor on theaccumulated charge value.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosed aspects will hereinafter be described in conjunction withthe appended drawings and appendices, provided to illustrate and not tolimit the disclosed aspects, wherein like designations denote likeelements.

FIG. 1A illustrates an embodiment of a pixel readout system.

FIG. 1B illustrates an embodiment of a pixel architecture for use as apixel of the pixel readout system of FIG. 1A.

FIG. 2 illustrates an embodiment of a four transistor 4-shared imagesensor pixel architecture having full well capacity extensioncapabilities.

FIG. 3 illustrates an embodiment of a four transistor 8-shared imagesensor pixel architecture having full well capacity extensioncapabilities.

FIG. 4 illustrates an embodiment of a four transistor 16-shared imagesensor pixel architecture having full well capacity extensioncapabilities.

FIG. 5A is a schematic diagram illustrating an embodiment of multiplereads for full well capacity extension.

FIG. 5B illustrates a flowchart of an embodiment of a kTC noisecompensation process implementing the reads of FIG. 5A.

FIG. 5C illustrates an example timing scheme for turning pixelarchitecture components on and off to perform the process of FIG. 5B.

FIG. 5D illustrates a graphical representation of charge flow betweenpixel architecture components during the timing scheme of FIG. 5C.

FIG. 6A is a schematic diagram illustrating an embodiment of multiplereads for full well capacity extension.

FIG. 6B illustrates a flowchart of an embodiment of a kTC noisecompensation process implementing the reads of FIG. 6A.

FIG. 7 illustrates a high-level schematic diagram of an embodiment of animage capture device with full well capacity extension capabilities.

FIG. 8A illustrates another embodiment of a pixel architecture for useas a pixel of the pixel readout system of FIG. 1A.

FIG. 8B illustrates another embodiment of a four transistor 4-sharedimage sensor pixel architecture having full well capacity extensioncapabilities.

FIG. 8C illustrates another embodiment of a four transistor 8-sharedimage sensor pixel architecture having full well capacity extensioncapabilities.

FIG. 8D illustrates another embodiment of a four transistor 16-sharedimage sensor pixel architecture having full well capacity extensioncapabilities.

FIG. 9A illustrates an example timing scheme for turning pixelarchitecture components on and off to perform an embodiment of a kTCnoise compensation process using the pixel architecture of any of FIGS.8A-8D.

FIG. 9B illustrates a graphical representation of charge flow betweenpixel architecture components during the timing scheme of FIG. 9A.

DETAILED DESCRIPTION Introduction

Embodiments of the disclosure relate to systems and techniques for fullwell capacity extension. For example, a pixel readout architectureincluding an extra storage capacitor can implement a signal readoutscheme to allow extension of full well capacity, boosting SNR anddynamic range. The storage capacitor and associated readout architecturecan be implemented, in various embodiments, with a four transistor4-shared CIS pixel, 8-shared, 16-shared, or other suitable pixelconfigurations. Pixel readout components including a timing scheme andthe storage capacitor can cooperate to store charges accumulated fromthe photo-detectors to perform multiple pixel readouts.

In one example, where a transfer gate, timing scheme, and resettransistor are all on, a photodiode and a floating diffusion can bereset. Charge integration in the storage capacitor can occur when thetransfer gate, timing scheme, and reset transistor are all off, allowingphoton charge to be collected in a photodiode. When timing circuitry isturned on one or more times, the charge in the photodiode can be dumpedto the storage capacitor. Accordingly, the storage capacitor can holdcharges from multiple reads of a pixel or group of pixels, extendingfull well capacity.

In some examples, the main source of noise within a CMOS pixel is kTC(or reset) noise from the resetting of the storage capacitor and/orcapacitive floating diffusion node through the resistive channel of thereset transistor (a few tens of electrons). One embodiment of a readoutmode that can be implemented using embodiments of the full well capacityextension architecture described herein can include a first readperformed right after reset by turning on the selector circuit. Thisfirst read can store the kTC (thermal noise) in the memory as abaseline. Such kTC noise can originate from the reset noise ofcapacitors, and it can be desirable for enhancing image quality toreduce, eliminate, or otherwise compensate for the kTC noise.Accordingly, a second readout can be performed after charge integrationin the storage capacitor (for example including multiple charge dumpsfrom a pixel or pixels) by turning on the selector circuit again. Due tothe additional charge storage capacity of the storage capacitor and themultiple photodiode charge dumps controlled by the timing scheme, thetwo readouts can provide multiplication of the pixel full well capacityproportionate to the number of charge dumps. Because the kTC noise wasstored in the memory as a baseline in the first frame, this readoutmethod can compensate for the kTC noise in the second readout. Byperforming two readouts, for example by sampling the storage capacitorbefore and after the transfer gate is turned on, correlated doublesampling with a short sampling time can be performed, thus eliminatingkTC noise.

Another embodiment of a readout mode that can be implemented usingembodiments of the full well capacity extension architecture describedherein can perform multiple readouts and obtain the charge differencebetween two charge dumps from a photodiode. The resulting differentialframe can be kTC noise free. In some embodiments, the multiple readoutscan be performed in a low bit depth mode to conserve power if highaccuracy is not needed, for example for a viewfinder preview image orfor auto-exposure adjustments. In some embodiments, the multiplereadouts can be performed in normal bit depth mode for more accurateimage information. In some embodiments, for example high dynamic range(HDR) imaging, two different integration times can be used for thecharge dumps in two readouts used to generate the differential frame.

Overview of Example Pixel Readout Architectures

FIG. 1A illustrates an embodiment of a pixel readout system 100including timing circuitry 111 (also referred to as a “timinggenerator,” a digital component though displayed within the analogcomponent block), a set of analog components 110 including a (sensor)pixel array 112, readout circuitry 114, amplifier 116, and analog todigital converter (ADC) 118, the components 110 coupled at a bus 120 toa memory 130. Though not illustrated, the bus 120 can enablecommunication with other components for example an image signalprocessor, device memory, and readout control modules.

The pixel readout system 100 can be used for any CMOS, CCD, or otherimage sensor. In some embodiments, the sensor can be, for example, a 32MP/30 fps image sensor having approximately 0.5 μm pixels withapproximately 1000-e full well capacity. The image sensor can bedesigned to have 10b/240-fps readout speed. Some embodiments of theimage sensor can be a digitally integrated design with 8-frameaccumulation shutter speed, for example, implementing an interleavedrolling shutter with no need to store a full frame. In one example, datacan be output from the external memory 120, for example DRAM, at speedsof approximately 12b/30-fps. Such an image sensor can result in anequivalent pixel full well capacity multiplied by a factor of eight, forexample reaching approximately 8000-e full well capacity per pixel.These image sensor specifications represent just one embodiment of animage sensor using the pixel readout system 100, and other image sensorswith varying specifications can be used in other embodiments.

The pixel array 112 may include a plurality of pixels 140 arranged in apredetermined number of rows and columns (e.g., M rows and N columns).Each pixel 140 represents a single photoelectric charge detectingelement of the pixel array 112. Each of the plurality of pixels can eachinclude a photosensitive element, for example a photogate,photoconductor, photodiode, or other photodetector, overlying asubstrate for accumulating photo-generated charge in an underlyingportion of the substrate. The pixel array 112 can, in some embodiments,include one or more filters positioned to filter incoming light, forexample an infrared cut-off filter or a color filter. The photosensitiveelement of a CMOS pixel can be one of a depleted p-n junction photodiodeor a field-induced depletion region beneath a photogate.

Timing generator 111 can provide timing signals for reading out valuesrepresenting light accumulated in each of the pixels of the pixel array112. For example, timing generator 111 can be a column and row driver.Timing generator can include control logic for controlling resetoperations, exposure time, line time, and pixel time for the pixel array112 and can also provide timing signals to the ADC 118. Readoutcircuitry 114 can provide circuitry for reading out each of the pixelsin the pixel array 112. For example, readout circuitry 114 can include aplurality of row and column lines provided for the entire array 112. Thecolumn and row lines of the readout circuitry 114 can be electronicallyconnected to column sample and hold (S/H) circuitry and row controlcircuitry, respectively, that operate according to the timing providedby the timing generator. In operation, the pixels of each row in thepixel array 112 can be turned on at the same time by a row select lineand the pixels of each column can be selectively output by a columnselect line.

Each pixel 140 includes circuitry for reading a charge collected in eachof the plurality of pixels. For example, one embodiment of the pixelcircuitry is connected to each pixel and includes an output field effecttransistor formed in the substrate and a charge transfer section formedon the substrate adjacent the pixel having a sensing node, typically afloating diffusion node, connected to the gate of an output transistor.A floating diffusion node (also referred to as just a floatingdiffusion) can be a region in active silicon that can be electricallyisolated from all other nodes (for example, when charge is being stored)and the potential of such a region is determined by the amount of chargestored in it and its capacitance. Capacitance of this region istypically quite low in order to achieve high conversion gain. The chargetransfer section of the pixel circuitry can include at least oneelectronic component, for example a transistor, for transferring chargefrom the underlying portion of the substrate to the floating diffusionnode and another electronic component, for example a reset transistorfor resetting the node to a predetermined charge level prior to chargetransference. Photo charge may be amplified when it moves from theinitial charge accumulation region to the floating diffusion node, andthe charge at the floating diffusion node can be converted to a pixeloutput voltage by a source follower output transistor. As discussed inmore detail below, the pixel circuitry can further include a storagecapacitor or other electronic charge storage device for storing multiplecharge dumps from a pixel or pixels in the analog domain. The storagecapacitor may be positioned beyond the floating diffusion node in thepixel circuitry (relative to the position of the pixel) in someembodiments, and can have a capacitance greater than that of thefloating diffusion node. The storage capacitor can be on the same sensorchip or in the bottom chip of a stack-die with pixel-to-pixel connectionin various embodiments.

After passing through the pixel circuitry, pixel signals are passedthrough the amplifier 116 to increase the strength of the pixel signal(i.e., the voltage or current). The timing generator 111, pixel array112, readout circuitry 114, and amplifier 116 together perform thefunctions of: (1) photon to charge conversion; (2) accumulation of imagecharge; (3) transfer of charge to the floating diffusion nodeaccompanied by charge amplification; (4) resetting the floatingdiffusion node to a known state before the transfer of charge to it; (5)selection of a pixel for readout; and (6) output and amplification of asignal representing pixel charge. In addition, the pixel readoutarchitecture for full well capacity extension described herein canperform functions (1) through (5) a number of times before output andamplification of the signal in order to accumulate charge in the storagecapacitor in the pixel circuitry, the accumulated charge being from anumber of charge dumps from one pixel or group of pixels. Theaccumulated charge can then be output and amplified as the signalrepresenting the pixel charge. Using the readout architectures for fullwell capacity extension described herein, pixel performance indices, forexample quantum efficiency, sensitivity, crosstalk, image lag,non-uniformity, and non-linearity can be optimized in the pixel designdisregarding full well capacity, which is extended through the analogreadout architecture.

The amplified pixel signal is passed through ADC 118 before being outputfrom the analog components 110. Because the pixel charges are read asanalog signals in the analog components 110, though digital signals anddigital circuits can offer advantages as compared to analog circuitswith respect to processing speed and efficient transmitting, the ADC 118converts the analog pixel charge signals into suitable digital signals,for example into corresponding digital pixel values. The ADC 118 canconvert an accumulated charge from the storage capacitor into acorresponding digital pixel value.

The digital components of the pixel readout system 100 can also functionto increase the full well capacity of the pixels in the pixel array 112.In some embodiments, multiple reads can be integrated in the digitaldomain with reasonable power budget, effectively multiplying the smallpixel's full well capacity, by leveraging embedded DRAM technology(e.g., by using DRAM as the memory 130). For example, several iterations(e.g., multiple frame capture) can be stored to memory 130. However, dueto the inclusion of the storage capacitor in the pixel circuitry,charges can be dumped from the photodiode to the storage capacitor anumber of times, reducing or eliminating the need for reading outmultiple times per frame, saving power. Multiple reads in the digitaldomain can be used in conjunction with multiple dumps to the storagecapacitor in the analog domain in some embodiments for additional fullwell capacity extension. In addition, by using N-iterations 135 oftransferring accumulated charge storage to the digital memory 130,output image data 125 can be modified to reduce or eliminate kTC noisethat can occur when resetting the storage capacitor. Memory 130 can beRAM, DRAM, flash memory, or any suitable type of memory for storingimage data in various embodiments.

For example, a technique for reducing or eliminating kTC noise can be tostore a kTC noise frame into memory as a baseline. The kTC noise can beread before dumping any charge into the storage capacitor to get itsreset level, and that reset level can be stored (for example in columnrow circuitry). A second read can be performed after dumping pixelcharges to the storage capacitor multiple times, where the second readis adjusted by the baseline. Accordingly, N-iterations can be twoiterations, still achieving the full well capacity extension benefitwhile reducing or eliminating the effects of kTC noise. Further, onlytwo iterations in the digital domain can be used to achieve the samefull well capacity extension as multiple reads in the digital domain bydumping charges multiple times to the analog storage capacitor.

As another example, a technique for reducing or eliminating kTC noisecan be to perform multiple reads and calculate the charge differencebetween two charge dumps from a photodiode. This may be kTC noise free.This technique can be used in some embodiments to read out in low bitdepth mode to save power if high accuracy is not required (for example,a quick auto-exposure adjustment can be performed based on thistechnique, or preview images can be generated based on this technique).In other embodiments, the technique can be used to read out in standardor high bit depth mode for more accurate image information.

In some embodiments, two different integration times can be used for thetwo charge dumps in the above-described techniques for HDR imaging. Insome embodiments, different integration times and/or different numbersof charge dumps to the storage capacitor can be used for differentpixels in the array 112 based, for example, on charge strength or pixelintensity value, in order to extend the dynamic range of the targetimage scene.

FIG. 1B illustrates one embodiment of the pixel circuitry 140 for onepixel of the pixel readout system 100 of FIG. 1A. Many such pixels 140are arranged in the array 112, for example forming rows and columns, butfor simplicity only one is shown in greater detail. The readoutcircuitry embodiment of FIG. 1B can perform both low noise and lag freecharge transfer as well as snap shot features, e.g., frame storage andcharge sink. In addition, the storage capacitor STO offers extended fullwell capacity for the pixel.

The pixel circuitry includes a photodiode PD for accumulatingphoto-generated charge. Charge collected in the photodiode PD can bepassed through transfer gate TX2 to the storage capacitor STO, and thestorage capacitor STO can be provided with an effective capacitancelarge enough to store multiple charge dumps from the photodiode PD, forexample eight or more charge dumps. The accumulated charge from thestorage capacitor STO can be passed through transfer gate TX1 to thefloating diffusion node FD. The selector transistor SEL allows a singlerow of the pixel array to be read by the image sensor. The sourcefollower SF_AMP converts the input applied from the floating diffusionnode FD into the output (e.g., voltage or current) sensed at the outputnode Vcol; in other words the source follower SF_AMP converts thedetected image charges into corresponding electrical signals.

The reset transistor RST acts as a switch to reset the floatingdiffusion FD. When the reset transistor RST is turned on, the photodiodePD is effectively connected to the power supply, clearing all integratedcharge. The pixel is reset when the reset transistor RST and transfergate TX3 are turned on simultaneously, setting both the floatingdiffusion FD and the photodiode PD to the VDD voltage level. Turning oneither of transfer gates TX2 or TX1 can set the storage capacitor STO tothe VDD voltage level as well. When the transfer gate TX3 is turned off,disconnecting the photodiode and floating diffusion, the photodiode PDis left to integrate light. In addition, TX3 serves as an overflow gatefor blooming protection control as well as enabling snap shot features(e.g., frame storage & charge sink), providing electronic globalshuttering operation so as to freeze fast action.

In one example, to implement the first kTC noise reduction techniquedescribed above, first, the reset transistor is turned on and off toreset the floating diffusion. Signal measurement of the reset level issampled from the floating diffusion and stored, for example on a columncircuit. Next, the transfer gate TG is turned on and off which allowscharge on the photodiode to transfer to the floating diffusion FD. Oncecharge transfer is complete, this charge (the photodiode signal levelplus the floating diffusion reset level) is measured and stored on thecolumn circuit as well. These two stored voltages are then differencedto determine the photodiode signal level, reducing kTC noise.

FIG. 2 illustrates an embodiment of a four transistor 4-shared imagesensor pixel architecture 200 having full well capacity extensioncapabilities. Some embodiments of the image sensor can be a solid-stateimage sensor, for example a CMOS image sensor, which has fourfour-transistor (4T) pixels sharing pixel circuitry and compact layout.The 4T 4-shared pixel architecture 200 can be used as the pixelarchitecture for pixels 140 in the pixel array 112 in some embodiments.The 4T 4-shared pixel architecture 200 includes four pixels, and thoughmany such pixels are arranged in the array 112, for simplicity only one4T 4-shared pixel architecture 200 is shown in greater detail.

The 4T 4-shared pixel architecture 200 includes four photodiodes PD1-PD4and their corresponding transfer gates TG1-TG4, a floating diffusionnode FD, reset transistor RST, voltage supply VDD, source followeramplifier SF_AMP, selector transistor SEL, output voltage node Vcol, andcurrent source Ibias. These elements can perform similar functions asdescribed above with respect to FIG. 1B. The 4T 4-shared pixelarchitecture 200 also includes timing circuitry TS connected in serieswith the floating diffusion FD and a storage capacitor CS connected inseries with the timing circuitry TS. Sharing of the floating diffusionFD, source follower amplifier SF_AMP, row select transistor SEL, timingcircuitry TS, reset transistor RST, and storage capacitor CS betweenadjacent photodiodes PD1-PD4 and transfer gates TG1-TG4 assists inincreasing the fill factor of the pixel architecture. The illustratedarchitecture 200 results in approximately 2.0 transistors per pixel.Components can be shared by four separate photodiodes and associatedtransfer gates. In some embodiments, photodiodes PD1-PD4 can include onephotodiode used to detect a red component of incoming light, twophotodiodes used to detect a green component of incoming light, and onephotodiode used to detect a blue component of incoming light.

Charge can be collected by each of the photodiodes PD1-PD4 as long asthe corresponding transfer gate TG1-TG4 is off, and the charge istransferred to the floating diffusion FD by turning on the correspondingtransfer gate TG1-TG4 after the integration time. The floating diffusionFD can be a parasitic capacitor inside the silicon wafer of thephotodiodes PD1-PD4, however this may not hold enough charge forextending the dynamic range of a sensor with small pixels. Accordingly,the charge from a photodiode PD1-PD4 can be read out multiple timesthrough the floating diffusion FD to the storage capacitor CS. Thestorage capacitor CS can be configured with an effective capacitancelarge enough to store multiple charge dumps from a photodiode, forexample eight or more charge dumps. In some embodiments, the capacitanceof the storage capacitor is greater than or equal to approximately 3.2femtofarads. Accordingly, the full well capacity of a pixel can beeffectively multiplied in the analog domain by the number of chargedumps stored in the storage capacitor CS.

In one example, where a transfer gate TG1-TG4, timing circuitry TS, andreset transistor RST are all on, a corresponding photodiode PD1-PD4 andthe floating diffusion FD can be reset. Charge integration can occurwhen the transfer gate TG1-TG4, timing circuitry TS, and resettransistor RST are all off, allowing photon charge to be collected in acorresponding photodiode PD1-PD4. By selectively turning on a transfergate TG1-TG4, the charge in a corresponding photodiode (e.g., the one ormore photodiodes having an associated transfer gate turned on) istransferred to the floating diffusion FD. When the timing circuitry TSis turned on one or more times, from the floating diffusion FD can bedumped to the storage capacitor CS. In some embodiments, one or moretransfer gates and the timing circuitry can be turned on simultaneouslyto transfer charge directly from associated photodiodes directly to thestorage capacitor CS.

During the charge readout of one of photodiodes PD1-PD4, all thephotoelectrons are mixed at the floating diffusion node FD andintegrated storage capacitor CS by turning both the correspondingtransfer gate of transfer gates TG1-TG4 and timing circuitry TS on. Thenthe signal is read out after turning the corresponding transfer gateoff. After turning the corresponding transfer gate off, the chargesensing node capacitance (that can be input to the SF_AMP's gate) is thecombination of capacitors of the floating diffusion FD, storagecapacitor CS, and timing circuitry TS. All the charges integrated at thephotodiode, floating diffusion FD, and storage capacitor CS are mixed upfollowed by readout via the source follower amplifier SF_AMP. Thisprocess performs charge-to-voltage conversion, and the V_FD and V_colrepresent the voltage level at FD and column video bus respectively. Thevoltage levels at the floating diffusion FD and at V_col may depend uponthe incident light level, that is, a number of photons incident on thephotodiodes PD1-PD4.

The timing circuitry TS can include high-speed logic for determiningwhich of the photodiodes PD1-PD4 to read for accumulation of charge inthe storage capacitor CS. The high-speed logic of the timing circuitryTS can also regulate the number of charge dumps sent from a photodiodesPD1-PD4 to the storage capacitor CS. By accumulating multiple chargedumps from a pixel in the analog domain, device power can be saved incomparison to accumulating multiple charge dumps from the pixel in thedigital domain. Sequential readout of photodiodes PD1-PD4 using thetiming circuitry TS can control the readout of photodiodes PD1-PD4 toincrease resolution in comparison to parallel scanning. Due to the highspeed logic of the timing circuitry TS, high video frame rates (e.g., 30fps at 8 MP full resolution) can still be achieved while sampling eachphotodiode in the pixel array multiple times in the analog domain, forexample eight readouts per pixel per frame. In some embodiments, thetiming scheme can be a transistor with a line weight of approximately 20nm.

One readout mode implemented using the architecture of FIG. 2 caninclude a first read performed right after reset by turning on theselector SEL circuit. This can store the kTC (thermal noise) in thememory as a baseline. A second readout can be performed after chargeintegration in the storage capacitor by turning on the selector circuitagain. Due to the additional capacity of the storage capacitor and themultiple photodiode charge dumps controlled by the timing scheme, thetwo readouts can provide multiplication of the pixel full well capacityproportionate to the number of charge dumps.

Another readout mode implemented using the architecture of FIG. 2 canperform multiple readouts and obtain the charge difference between two“charge dumps” from a photodiode. The resulting differential frame canbe kTC noise free. The multiple readouts can be performed in a low bitdepth mode to save power if high accuracy is not needed, for example fora viewfinder preview image or for auto-exposure adjustments. Themultiple readouts can be performed in normal bit depth mode for moreaccurate image information. In some embodiments, for example highdynamic range (HDR) imaging, two different integration times can be usedfor the charge dumps in two readouts used to generate the differentialframe.

FIG. 3 illustrates an embodiment of a four transistor 8-shared imagesensor pixel architecture 300 having full well capacity extensioncapabilities. Some embodiments of the image sensor can be a solid-stateimage sensor, for example a CMOS image sensor, which has eight 4T pixelssharing pixel circuitry and compact layout. The 4T 8-shared pixelarchitecture 300 can be used as the pixel architecture for pixels in thepixel array 112 in some embodiments. The 4T 8-shared pixel architecture300 includes eight pixels, and though many such pixels are arranged inthe array 112, for simplicity only one 4T 8-shared pixel architecture300 is shown in greater detail.

The 4T 8-shared pixel architecture 300 includes eight photodiodesPD1-PD8 and their corresponding transfer gates TG1-TG8, a floatingdiffusion node FD, reset transistor RST, timing circuitry TS, storagecapacitor CS, voltage supply VDD, source follower amplifier SF_AMP,selector transistor SEL, output voltage node Vcol, and current sourceIbias. These components can perform similar functions as described abovewith respect to FIG. 2. These components can be shared by eight separatephotodiodes and associated transfer gates. Sharing of the floatingdiffusion FD, source follower amplifier SF_AMP, row select transistorSEL, timing circuitry TS, reset transistor RST, and storage capacitor CSbetween adjacent photodiodes PD1-PD8 and transfer gates TG1-TG8 assistsin increasing the fill factor of the pixel architecture. The illustratedarchitecture 300 results in approximately 1.5 transistors per pixel. Insome embodiments, photodiodes PD1-PD8 can include two photodiodes usedto detect a red component of incoming light, four photodiodes used todetect a green component of incoming light, and two photodiodes used todetect a blue component of incoming light.

As described above, the timing circuitry TS can include high-speed logicconfigured to control accumulation of one or more charge dumps from aphotodiode PD1-PD8 in the storage capacitor CS for power-efficientextension of the small pixel's full well capacity in the analog domain.The accumulated charge can be read out from the storage capacitor CSthrough the source follower SF_AMP to the output voltage node Vcol.

FIG. 3 also illustrates one embodiment of partitioning of the pixelarray and read-out circuitry into separate, stacked structures (e.g.,silicon wafers). For example, a first portion 310 including thephotodiodes PD1-PD8, the corresponding transfer gates TG1-TG8, and thefloating diffusion node FD can be placed in a photodetector waferconfigured to integrate charge from incoming light. A second portion 320including the reset transistor RST, timing circuitry TS, storagecapacitor CS, voltage supply VDD, source follower amplifier SF_AMP,selector transistor SEL, output voltage node Vcol, and current sourceIbias can be placed in a pixel readout circuitry wafer. Accordingly, thesurface space for detecting light can be increased through both thespace-saving design of the shared pixel architecture as well as theseparation of the photodiodes and the pixel readout circuitry intodifferent wafers. The timing circuitry TS and source follower amplifierSF_AMP of the pixel readout circuitry wafer can be connected to thefloating diffusion FD of the photodetector wafer.

FIG. 4 illustrates an embodiment of a four transistor 16-shared imagesensor pixel architecture 400 having full well capacity extensioncapabilities. Some embodiments of the image sensor can be a solid-stateimage sensor, for example a CMOS image sensor, which has sixteen 4Tpixels sharing pixel circuitry and compact layout. The 4T 16-sharedpixel architecture 400 can be used as the pixel architecture for pixelsin the pixel array 112 in some embodiments. The 4T 16-shared pixelarchitecture 400 includes sixteen pixels, and though many such pixelsare arranged in the array 112, for simplicity only one 4T 16-sharedpixel architecture 400 is shown in greater detail.

The 4T 16-shared pixel architecture 400 includes sixteen photodiodesPD1-PD16 and their corresponding transfer gates TG1-TG16, a floatingdiffusion node FD, reset transistor RST, timing circuitry TS, storagecapacitor CS, voltage supply VDD, source follower amplifier SF_AMP,selector transistor SEL, output voltage node Vcol, and current sourceIbias. These elements can perform similar functions as described abovewith respect to FIG. 2. These components can be shared by the sixteenseparate photodiodes and associated transfer gates. Sharing of thefloating diffusion FD, source follower amplifier SF_AMP, row selecttransistor SEL, timing circuitry TS, reset transistor RST, and storagecapacitor CS between adjacent photodiodes PD1-PD16 and transfer gatesTG1-TG16 assists in increasing the fill factor of the pixelarchitecture. The illustrated architecture 400 results in approximately1.0 transistors per pixel. In some embodiments, photodiodes PD1-PD16 caninclude four photodiodes to detect a red component of incoming light,eight photodiodes to detect a green component of incoming light, andfour photodiodes to detect a blue component of incoming light.

As described above, the timing circuitry TS can include high-speed logicconfigured to control accumulation of one or more charge dumps from aphotodiode PD1-PD16 in the storage capacitor CS for power-efficientextension of the small pixel's full well capacity in the analog domain.The accumulated charge can be read out from the storage capacitor CSthrough the source follower SF_AMP to the output voltage node Vcol.

FIG. 4 also illustrates one embodiment of partitioning of the pixelarray and read-out circuitry into separate, stacked structures (e.g.,silicon wafers). For example, a first portion 410 including thephotodiodes PD1-PD16, the corresponding transfer gates TG1-TG16, and thefloating diffusion node FD can be placed in a photodetector waferconfigured to integrate charge from incoming light. A second portion 420including the reset transistor RST, timing circuitry TS, storagecapacitor CS, voltage supply VDD, source follower amplifier SF_AMP,selector transistor SEL, output voltage node Vcol, and current sourceIbias can be placed in a pixel readout circuitry wafer. Accordingly, thesurface space for detecting light can be increased through both thespace-saving design of the shared pixel architecture as well as theseparation of the photodiodes and the pixel readout circuitry intodifferent wafers. The timing circuitry TS and source follower amplifierSF_AMP of the pixel readout circuitry wafer can be connected to thefloating diffusion FD of the photodetector wafer.

FIGS. 1B, 2, 3, and 4 illustrate various pixel circuitry architecturesthat can be used to extend the full well capacity of small pixels. Insome embodiments, having even numbers of shared pixels and/or multiplesof four shared pixels can provide for a more compact image sensorlayout, pixel symmetry, uniformity of pixel structure, and noisesuppression. These examples are provided to illustrate, and not tolimit, the full well capacity extension capabilities of the storagecapacitor CS and timing circuitry TS. In other embodiments, othernumbers of shared pixels (e.g., 32) or other pixel architecturesincluding fewer or greater numbers of transistors per pixel (e.g., 3T,5T, etc.) can be used together with the storage capacitor CS and timingcircuitry TS to extend pixel full well capacity in the analog domain.

Overview of Example Pixel Readout Techniques

FIG. 5A is a schematic diagram illustrating an embodiment of multiplereads for full well capacity extension. Frame-1 501 represents thebaseline kTC noise of a storage capacitor, for example by reading outthe charge in a storage capacitor in the pixel readout architectureafter reset. Frame-2 502 represents image data acquired through aplurality of charge dumps 503 from each pixel, for example by using thestorage capacitor in the pixel readout architecture to enableaccumulation of a plurality of charge dumps 503 from each pixel in theanalog domain.

FIG. 5B illustrates a flowchart of an embodiment of a kTC noisecompensation process 500 implementing the readouts of FIG. 5A. In someembodiments of the readout architectures described above, the mainsource of noise within a pixel is kTC (or reset) noise from theresetting of the capacitive floating diffusion node through theresistive channel of the reset transistor. Such kTC noise can range froma few electrons to 100 electrons or more, and can accordingly affect thevalue read from the pixels of an image sensor. The kTC noisecompensation process 500 represents one embodiment of a readout modethat can be implemented using embodiments of the full well capacityextension architectures described herein to minimize or eliminate theeffects of kTC noise.

FIG. 5C illustrates an example timing scheme 540 with six timing pointst1-t6 for turning pixel architecture components on and off to performthe process of FIG. 5B. The timing scheme 540 can be used to accomplisha pixel readout process having six steps in accordance with the timingpoints: (t1) Reset photo-detector/FD/CS; (t2) Reset off, light exposure,& charge integration starts; (t3) During charge integration of exposure,non-saturated charges stay in PD, & all saturated charges overflow to FD& CS via TG & TS; (t4) After the integration, TS is shut off; signalcharges are spilt into FD & CS; (t5) Photon-generated charges aretransferred to FD as TG switched on, & readout after TG is set off; and(t6) Multiple-dump signal charges occurs between t5 & t6, and allcharges are collected at the FD+CS. FIG. 5D illustrates graphicalrepresentations of charge flow between pixel architecture componentscorresponding to the timing points t1-t6 of the timing scheme 540 ofFIG. 5C. Accordingly, FIGS. 5C and 5D will be discussed in conjunctionwith the process 500 of FIG. 5B.

At block 505, the process 500 resets a photodiode and floating diffusionnode in the pixel array and pixel circuitry, for example by turning on atransfer gate, timing scheme, and reset transistor. This effectivelyclears any integrated charge from the photodiode and floating diffusionnode. Block 505 corresponds to the timing point t1 in the timing scheme540 and the charge flow diagram labeled “@t1” of FIG. 5D. Asillustrated, the reset transistor RST, transfer gate TG, and timingcircuitry TS, and selector SEL are turned on so all charge in thephotodiode PD, floating diffusion FD, and storage capacitor CS can flowto the voltage level Vdd.

At block 510, the charge in the storage capacitor and floating diffusionconnected in parallel is read out through the floating diffusion node(e.g., frame-1 501) by turning on the selector circuit. This read canstore the kTC in a memory as a baseline. Block 510 corresponds to thetiming point t2 in the timing scheme 540 and the charge flow diagramlabeled “@t2” of FIG. 5D. As illustrated, the reset transistor RST,timing circuitry TS, and selector SEL are on and the transfer gate TG isoff so that the charge in the floating diffusion FD and storagecapacitor CS can be sampled.

At block 515, the process 500 allows photon charge collection in aphotodiode for a determined integration time, often referred to as theexposure time. For example, the transfer gate, timing circuitry, andreset transistor associated with the photodiode can be turned off toallow photon charge collection in the photodiode. In a CMOS sensor, theintegration time can be dynamically selected per pixel to extend dynamicrange in some embodiments. Block 515 corresponds to the timing point t3in the timing scheme 540 and the charge flow diagram labeled “@t3” ofFIG. 5D. As illustrated, the reset transistor RST, transfer gate TG, andselector SEL are off and the timing circuitry TS is on so charge cancollect in the photodiode PD. As illustrated in FIG. 5D, anyoversaturated charge from the photodiode PD spills over into thefloating diffusion FD and storage capacitor CS instead of intoneighboring pixels.

At block 520, the process 500 can initiate charge dump from thephotodiode to the storage capacitor. For example, in some embodimentsthe transfer gate associated with the photodiode can be turned on totransfer the charge to the storage capacitor. In some embodiments, thetransfer gate associated with the photodiode can be turned on totransfer the charge to the floating diffusion, and then the timingcircuitry can be turned on to transfer the charge from the floatingdiffusion to the storage capacitor. In some embodiments, the transfergate associated with the photodiode and the timing circuitry can beturned on simultaneously to transfer the charge from the photodiodethrough the floating diffusion to the storage capacitor. Block 515corresponds to the timing points t4-t6 in the timing scheme 540 and thecharge flow diagrams labeled “@t4,” “@t5,” and “@t6” of FIG. 5D. Asillustrated, at timing point t4 the reset transistor RST, timingcircuitry TS, and transfer gate TG are all off. At timing point t5 thetransfer gate TG is turned on to allow charge to flow from thephotodiode PD to the floating diffusion FD. At timing point t6 thetiming circuitry t6 is turned on to allow charge to flow between thefloating diffusion FD and storage capacitor CS connected in parallel,such that the storage capacitor CS allows for collection of a greateramount of charge. Timing points t3 through t6 can be repeated for thedesired number of charge dumps.

At block 525, the process 500 can determine whether the number of chargedumps from the photodiode to the storage capacitor is equal to N. Insome embodiments, N represents a predetermined number of charge dumps,for example 2, 4, 8, or more, depending on the full well capacityextension needed for the pixels of the image sensor. In someembodiments, N can be a dynamically determined number of charge dumps,for example based on a charge value corresponding to a desired intensityrange for the pixels of the image sensor. The number N of charge dumpsis controlled via the timing circuitry TS, and is performed betweentiming point t5 and timing point t6 in the pixel operation timingdiagram. In some examples, N can be determined by Automatic ExposureControl (AEC) in sensor timing control logic.

If, at block 525, the process 500 determines that the number of chargedumps is equal to N, then the process 500 loops back to block 515 toallow photon charge collection in the photodiode for a determinedintegration time. The process 500 transitions through blocks 520 and 525to perform the charge dump to the storage capacitor as described above.

If, at block 525, the process 500 determines that the number of chargedumps is equal to N, then the process 500 transitions to block 530 tomeasure the charge from the storage capacitor (e.g., frame-2 502) byturning on the selector circuit again. Accordingly, this second readoutcan be performed after charge integration in the storage capacitorincluding accumulation of multiple charge dumps from a pixel. Due to theadditional charge storage capacity of the storage capacitor and themultiple photodiode charge dumps controlled by the timing scheme, theframe-2 502 readout can provide multiplication of the pixel full wellcapacity proportionate to the number N of charge dumps. The reset valueof the storage capacitor (frame-1 501) can be removed from the signalvoltage of the pixel (frame-2 502). Because the kTC noise was stored inthe memory as a baseline in the first frame, this readout method cancompensate for (e.g., reduce or eliminate) the kTC noise in the secondreadout. The floating diffusion FD serves as the in-pixel memory toenable analog correlated double sampling (CDS) in a pixel cell notincluding the timing circuitry TS and the storage capacitor CS. Theaddition of the timing circuitry TS and storage capacitor CS induceadditional reset kTC noise, and some embodiments rely on the digital CDSdescribed herein to eliminate such additional kTC noise.

FIG. 6A is a schematic diagram illustrating an embodiment of multiplereads for full well capacity extension. As described above with respectto FIGS. 5A and 5B, frame-1 501 represents the baseline kTC noise of astorage capacitor, for example by reading out the charge in a storagecapacitor in the pixel readout architecture after reset. Frame-2 502represents image data acquired through a plurality of charge dumps 503from each pixel, for example by using the storage capacitor in the pixelreadout architecture to enable accumulation of a plurality of chargedumps 503 from each pixel in the analog domain. Difference 601represents the differential frame calculated by subtracting the kTCnoise as represented by the value of frame-1 501 from the accumulatedintegrated charge value as represented by frame-2 502.

FIG. 6B illustrates a flowchart of an embodiment of a kTC noisecompensation process 600 implementing the readouts of FIG. 6A. Blocks605 through 625 correspond to blocks 505 through 525, respectively, ofprocess 500 as described above for obtaining values for frame-1 501 andframe-2 502, where the value of frame-2 502 represents N charge dumpsfrom a pixel into the storage capacitor. Accordingly, the descriptionwill not be repeated here for brevity.

At block 630, the process 600 can measure the charge from the storagecapacitor to determine the value of frame-2 502 by turning on theselector circuit again. This value of frame-2 502 may be stored to thememory or transmitted together with frame-1 501 for calculation of thedifferential frame 601.

At block 635, the process 600 can calculate the value of thedifferential frame 601 by subtracting the value of the kTC noise asrepresented by frame-1 501 from the signal voltage of the pixel (e.g.,accumulated integrated charge dumps from the storage capacitor) asrepresented by frame-2 502. Accordingly, the differential frame 601 canbe free or substantially free from kTC noise.

At block 640, the process 600 can store the differential frame in anexternal memory. For example, the external memory can be the memory 130of the pixel readout system 100 described above.

At block 645, the number of differential frames stored in the externalmemory can be compared to a threshold M. The threshold M can represent apredetermined or dynamically determined number of differential frames invarious embodiments. If, at block 645, the process 600 determines thatthe number of differential frames is less than M, then the process 600can loop back through blocks 605 through 640 to generate an additionaldifferential frame for storage in the memory.

If, at block 645, the process 600 determines that the number ofdifferential frames is equal to M, then the process 600 can transitionto block 650. At block 650, the process 600 (for example, by using animage signal processor) may aggregate the differential frames to furtherextend the pixel full well capacity. This further extension of the fullwell capacity occurs in the digital domain (for example, after thephotodiode charge has been converted from an analog to a digital value),in contrast to the full well capacity extension in the analog domainprovided by the multiple charge dumps to the storage capacitor.

In some embodiments, the M differential frames can be generated in a lowbit depth mode to conserve power if high accuracy is not needed, forexample for a viewfinder preview image or for auto-exposure adjustments.In some embodiments, the M differential frames can be generated innormal bit depth mode for more accurate image information. In someembodiments, M=2 and accordingly two differential frames are captured.The two differential frames can be captured using different integrationtimes, which can provide advantages in some imaging applications, forexample high dynamic range (HDR) imaging.

Overview of Example Imaging Device

FIG. 7 illustrates a high-level schematic diagram of an embodiment of animage capture device 700 with full well capacity extension capabilities,the device 700 having a set of components including an image processor720 linked to a camera 701 (image sensor). The image processor 720 isalso in communication with a working memory 765, memory 730, and deviceprocessor 755, which in turn is in communication with storage 770 and anoptional electronic display 760.

Device 700 may be a portable personal computing device, e.g. a mobilephone, digital camera, tablet computer, personal digital assistant, orthe like. There are many portable computing devices in which using thefull well capacity extension techniques as described herein wouldprovide advantages, particularly as the image sensors in such devicestrend toward smaller pixels with more limited full well capacity. Device700 may also be a stationary computing device or any device in which thefull well capacity extension techniques would be advantageous. Aplurality of applications may be available to the user on device 700.These applications may include traditional photographic and videoapplications as well as applications specific to full well capacityextension, for example applications relating to user management ofnumber of charge dumps, number of differential frames, HDR imagingapplications, and the like.

The image capture device 700 includes camera 701 for capturing externalimages. The camera 701 can include a pixel array 710 and pixel circuitrywith additional storage capacitor 715 as described above. In accordancewith the full well capacity extending architectures described above, thestorage capacitor can have the capacitance to store a number of chargedumps from one or more pixels, thereby extending pixel full wellcapacity in the analog domain. The camera 701 can be configured forcontinuous or intermittent capture of preview frames, as well as captureof full resolution final images.

The image processor 720 may be configured to perform various processingoperations on received preview frames in order to execute full wellcapacity extension techniques. Processor 720 may be a general purposeprocessing unit or a processor specially designed for imagingapplications. Examples of image processing operations include AWB andAEC data generation, LED current calculation, cropping, scaling (e.g.,to a different resolution), image stitching, image format conversion,color interpolation, color processing, image filtering (e.g., spatialimage filtering), lens artifact or defect correction, etc. Processor 720may, in some embodiments, comprise a plurality of processors. Processor720 may be one or more dedicated image signal processors (ISPs) or asoftware implementation of a processor.

As shown, the image processor 720 is connected to a memory 730 and aworking memory 765. In the illustrated embodiment, the memory 730 storescapture control module 735, full well capacity extension module 740,noise compensation module 780, and operating system 750. The modules ofthe memory 730 include instructions that configure the image processor720 of device processor 755 to perform various image processing anddevice management tasks. Working memory 765 may be used by imageprocessor 720 to store a working set of processor instructions containedin the modules of memory 730. Alternatively, working memory 765 may alsobe used by image processor 720 to store dynamic data created during theoperation of device 700.

Full well capacity extension module 740 can store sub-modules timingscheme module 742 and charge dump number handler module 744. Together,these modules can cooperate to perform the tasks relating to determiningeach pixel's integration time and multiple charge dumps from the pixelto the storage capacitor in the readout architecture.

Noise compensation module 780 can store sub-modules frame capture module782 and digital aggregation module 784. In some embodiments, framecapture module 782 can be provided with instructions that configure theprocessor 720 to perform the process 500 described above by providinginstructions to the pixel circuitry of the image sensor. In someembodiments, digital aggregation module 784 can be provided withinstructions that configure the processor 720 to perform the process 600described above to aggregate multiple charges in the digital domain.

As mentioned above, the image processor 720 is configured by severalmodules stored in the memories. The capture control module 735 mayinclude instructions that configure the image processor 720 to adjustthe focus position of camera 701. Capture control module 735 may furtherinclude instructions that control the overall image capture functions ofthe device 700. For example, capture control module 735 may includeinstructions that call subroutines to configure the image processor 720to capture preview image data or full resolution image data includingone or more frames of a target image scene using the camera 701.

Operating system module 750 configures the image processor 720 to managethe working memory 765 and the processing resources of device 700. Forexample, operating system module 750 may include device drivers tomanage hardware resources for example the camera 701. Therefore, in someembodiments, instructions contained in the image processing modulesdiscussed above may not interact with these hardware resources directly,but instead interact through standard subroutines or APIs located inoperating system component 750. Instructions within operating system 750may then interact directly with these hardware components. Operatingsystem module 750 may further configure the image processor 720 to shareinformation with device processor 755.

Device processor 755 may be configured to control the display 760 todisplay the captured image, or a preview of the captured image, to auser. The display 760 may be external to the imaging device 700 or maybe part of the imaging device 700. The display 760 may also beconfigured to provide a view finder displaying a preview image for a useprior to capturing an image, for example present the user with a visualrepresentation of the dynamic range of the image scene or with a userinterface for manually adjusting the number of analog charge dumpsand/or digitally accumulated differential frames. The display 760 maycomprise an LCD or LED screen, and may implement touch sensitivetechnologies.

Device processor 755 may write data to storage module 770, for exampledata representing digitally accumulated differential frames. Whilestorage module 770 is represented graphically as a traditional diskdevice, those with skill in the art would understand that the storagemodule 770 may be configured as any storage media device. For example,the storage module 770 may include a disk drive, e.g. a floppy diskdrive, hard disk drive, optical disk drive or magneto-optical diskdrive, or a solid state memory e.g. a FLASH memory, RAM, ROM, and/orEEPROM. The storage module 770 can also include multiple memory units,and any one of the memory units may be configured to be within the imagecapture device 700, or may be external to the image capture device 700.For example, the storage module 770 may include a ROM memory containingsystem program instructions stored within the image capture device 700.The storage module 770 may also include memory cards or high speedmemories configured to store captured images which may be removable fromthe camera. The storage module 770 can also be external to device 700,and in one example device 700 may wirelessly transmit data to thestorage module 770, for example over a network connection.

Although FIG. 7 depicts a device having separate components to include aprocessor, imaging sensor, and memory, one skilled in the art wouldrecognize that these separate components may be combined in a variety ofways to achieve particular design objectives. For example, in analternative embodiment, the memory components may be combined withprocessor components, for example to save cost and/or to improveperformance.

Additionally, although FIG. 7 illustrates two memory components,including memory component 720 comprising several modules and a separatememory 765 comprising a working memory, one with skill in the art wouldrecognize several embodiments utilizing different memory architectures.For example, a design may utilize ROM or static RAM memory for thestorage of processor instructions implementing the modules contained inmemory 730. The processor instructions may be loaded into RAM tofacilitate execution by the image processor 720. For example, workingmemory 765 may comprise RAM memory, with instructions loaded intoworking memory 765 before execution by the processor 720.

Overview of Additional Pixel Architecture and Readout Examples

In some embodiments, the above-described pixel architectures may facechallenges in supporting multiple dumps for different pixels. Forinstance, a shared architecture may have multiple pixels spillingcharges onto the floating diffusion and storage capacitor combination.In some instances the signals can be combined and it can be problematicto distinguish the charge contribution from each pixel. For example,when multiple pixels share the floating diffusion node and storagecapacitor, it can be difficult to distinguish charge contribution fromindividual pixels in saturation conditions. Further in someimplementations of the shared-pixel cells of FIGS. 2-4, signal may belost during multiple reads on the floating diffusion node. Accordingly,a number of implementations are proposed in FIGS. 8A-8D for improvedsupport of full well capacity extension by providing a storage capacitoruniquely associated with each photodiode. For example, suchimplementations can include a storage capacitor positioned between eachphotodiode and the corresponding timing circuitry and floating diffusionnode.

FIG. 8A illustrates another embodiment of a pixel architecture for useas a pixel of the pixel readout system of FIG. 1A. Many such pixels 800Aare arranged in the array 112, for example forming rows and columns, butfor simplicity only one is shown in greater detail. The pixelarchitecture embodiment of FIG. 8A can perform both low noise and lagfree charge transfer as well as snap shot features, e.g., frame storageand charge sink. In addition, the storage capacitor CS in combinationwith the floating diode FD offers extended full well capacity for thepixel 800A.

The pixel circuitry includes a photodiode PD for accumulatingphoto-generated charge. Charge collected in the photodiode PD can bepassed through transfer gate TG to the storage capacitor CS, and thestorage capacitor CS can be provided with an effective capacitance largeenough to store multiple charge dumps from the photodiode PD, forexample eight or more charge dumps. The accumulated charge from thestorage capacitor CS can be passed through timing circuitry transistorTS to the floating diffusion node FD. When the timing circuitry TS isturned on, the storage capacitor CS and floating diode FD can share thecharge accumulated from the photodiode PD. The selector transistor SELallows a single row of the pixel array to be read by the image sensor.The source follower SF_AMP converts the input applied from the storagecapacitor CS and floating diffusion node FD into the output (e.g.,voltage or current) sensed at the output node Vcol; in other words thesource follower SF_AMP converts the detected image charges intocorresponding electrical signals.

The reset transistor RST acts as a switch to reset the floatingdiffusion FD and storage capacitor CS. When the reset transistor RST isturned on, the photodiode PD is effectively connected to the powersupply, clearing all integrated charge. The pixel is reset when thereset transistor RST, transfer gate TG, and timing circuitry TS areturned on simultaneously, setting the floating diffusion FD, storagecapacitor CS, and the photodiode PD to the VDD voltage level. When thetransfer gate TG is turned off, disconnecting the photodiode PD andstorage capacitor CS, the photodiode PD is left to integrate light. Thetiming circuitry TS can serve as an overflow gate for bloomingprotection control as well as enabling snap shot features (e.g., framestorage & charge sink), providing electronic global shuttering operationso as to freeze fast action.

In one example, to implement a kTC noise reduction technique asdescribed above, first, the reset transistor RST is turned on and off toreset the floating diffusion FD and storage capacitor CS. Signalmeasurement of the reset level is sampled from the floating diffusion FDand storage capacitor CS with the timing scheme TS turned on and stored,for example on a column circuit. Next, the transfer gate TG is turned onand off which allows charge on the photodiode PD to transfer to thestorage capacitor CS. When the timing scheme TS is turned on, thestorage capacitor CS and floating diffusion FD are connected inparallel. Once charge transfer is complete, this charge (the photodiodesignal level plus the floating diffusion reset level) is measured andstored on the column circuit as well. These two stored voltages are thendifferenced to determine the photodiode signal level, reducing kTCnoise.

FIG. 8B illustrates an embodiment of a 4-shared image sensor pixelarchitecture 800B having full well capacity extension capabilities. Someembodiments of the image sensor can be a solid-state image sensor, forexample a CMOS image sensor, which has four pixels sharing pixelcircuitry and compact layout. The 4-shared pixel architecture 800B canbe used as the pixel architecture for pixels 140 in the pixel array 112in some embodiments. The 4-shared pixel architecture 800B includes fourpixels, and though many such pixels are arranged in the array 112, forsimplicity only one pixel architecture 800B is shown in greater detail.

The 4-shared pixel architecture 800B includes four photodiodes PD1-PD4and their corresponding transfer gates TG1-TG4, a floating diffusionnode FD, reset transistor RST, voltage supply VDD, source followeramplifier SF_AMP, selector transistor SEL, output voltage node Vcol, andcurrent source Ibias. These elements can perform similar functions asdescribed above with respect to FIG. 8A. The 4-shared pixel architecture800B also includes a storage capacitor CS1-CS4 and corresponding timingcircuitry TS1-TS4 connected in series between each of transfer gatesTG1-TG4 and the floating diffusion FD. Sharing of the floating diffusionFD, source follower amplifier SF_AMP, row select transistor SEL, andreset transistor RST, between adjacent photodiodes PD1-PD4 assists inincreasing the fill factor of the pixel architecture. In someembodiments, photodiodes PD1-PD4 can include one photodiode used todetect a red component of incoming light, two photodiodes used to detecta green component of incoming light, and one photodiode used to detect ablue component of incoming light.

Charge can be collected by each of the photodiodes PD1-PD4 as long asthe corresponding transfer gate TG1-TG4 is off. Multiple integrations ofthe charge in a photodiode PD1-PD4 can be sent to the associated storagecapacitor CS1-CS4 when the associated transfer gate TG1-TG4 is turnedon. The accumulated charge is transferred to the floating diffusion FDby turning on the corresponding timing circuitry TS1-TS4 after thepredetermined number of integration times. The floating diffusion FD canbe a parasitic capacitor inside the silicon wafer of the pixelarchitecture 800B, however this may not hold enough charge for extendingthe dynamic range of a sensor with small pixels. Accordingly, the chargefrom a photodiode PD1-PD4 can be read out multiple times through theassociated storage capacitor CS1-CS4 to the floating diffusion FD. Thestorage capacitor CS1-CS4 can be configured with an effectivecapacitance large enough to store multiple charge dumps from aphotodiode, for example eight or more charge dumps. In some embodiments,the capacitance of the storage capacitor CS1-CS4 is greater than orequal to approximately 3.2 femtofarads. Accordingly, the full wellcapacity of a pixel can be effectively multiplied in the analog domainby the number of charge dumps stored in the storage capacitor CS1-CS4.

In one example, where a transfer gate TG1-TG4, its associated timingcircuitry TS1-TS4, and the reset transistor RST are all on, acorresponding photodiode PD1-PD4, its storage capacitor CS1-CS4, and thefloating diffusion FD can be reset to the VDD voltage level. Chargeintegration can occur when the transfer gate TG1-TG4 and resettransistor RST are off, allowing photon charge to be collected in acorresponding photodiode PD1-PD4. By selectively turning on a transfergate TG1-TG4, the charge in a corresponding photodiode is transferred tothe storage capacitor CS1-CS4 connected to the photodiode PD1-PD4. Whenthe timing circuitry TS1-TS4 is turned on one or more times, the chargestored in storage capacitor CS1-CS4 can flow to the floating diffusionFD. In some embodiments, a transfer gate TG1-TG4 and the associatedtiming circuitry TS1-TS4 can be turned on simultaneously to transfercharge directly from associated photodiodes directly to the storagecapacitor CS1-CS4 and floating diffusion FD.

During the charge readout of one of photodiodes PD1-PD4, all thephotoelectrons are mixed at the floating diffusion node FD and anassociated one of the integrated storage capacitors CS1-CS4. Then thesignal is read out after turning the corresponding transfer gate TG1-TG4off. After turning the corresponding transfer gate off, the chargesensing node capacitance (that can be input to the SF_AMP's gate) is thecombination of capacitors of the floating diffusion FD, one of thestorage capacitors CS1-CS4, and the timing circuitry TS1-TS4 connectingthe floating diffusion FD and the one storage capacitor. All the chargesintegrated at the photodiode, floating diffusion FD, and storagecapacitor are mixed up followed by readout via the source followeramplifier SF_AMP. This process performs charge-to-voltage conversion,and the V_FD and V_col represent the voltage level at FD and columnvideo bus respectively. The voltage levels at the floating diffusion FDand at V_col may depend upon the incident light level, that is, a numberof photons incident on the photodiodes PD1-PD4.

The timing circuitry TS1-TS4 can include high-speed logic for a numberof times to read an associated one of the photodiodes PD1-PD4 to readfor accumulation of charge in the connected storage capacitor CS1-CS4.The high-speed logic of the timing circuitry can regulate the number ofcharge dumps sent from a photodiode to the storage capacitor CS. Byaccumulating multiple charge dumps from a pixel in the analog domain,device power can be saved in comparison to accumulating multiple chargedumps from the pixel in the digital domain. Sequential readout ofphotodiodes PD1-PD4 using the timing circuitry TS1-TS4 can control thereadout of photodiodes PD1-PD4 to increase resolution in comparison toparallel scanning. Due to the high speed logic of the timing circuitry,high video frame rates (e.g., 30 fps at 8 MP full resolution) can stillbe achieved while sampling each photodiode in the pixel array multipletimes in the analog domain, for example eight readouts per pixel perframe. In some embodiments, each of timing schemes TS1-TS4 can be atransistor with a line weight of approximately 20 nm.

One readout mode implemented using the architecture of FIG. 8B caninclude a first read performed right after reset by turning on theselector SEL circuit. This can store the kTC (thermal noise) in thememory as a baseline. A second readout can be performed after chargeintegration in the storage capacitor and floating diode by turning onthe selector circuit again. Due to the additional capacity of thestorage capacitor and the multiple photodiode charge dumps controlled bythe timing scheme, the two readouts can provide multiplication of thepixel full well capacity proportionate to the number of charge dumps.

Another readout mode implemented using the architecture of FIG. 8B canperform multiple readouts and obtain the charge difference between two“charge dumps” from a photodiode. The resulting differential frame canbe kTC noise free. The multiple readouts can be performed in a low bitdepth mode to save power if high accuracy is not needed, for example fora viewfinder preview image or for auto-exposure adjustments. Themultiple readouts can be performed in normal bit depth mode for moreaccurate image information. In some embodiments, for example highdynamic range (HDR) imaging, two different integration times can be usedfor the charge dumps in two readouts used to generate the differentialframe.

FIG. 8C illustrates an embodiment of an 8-shared image sensor pixelarchitecture having full well capacity extension capabilities. Someembodiments of the image sensor can be a solid-state image sensor, forexample a CMOS image sensor, which has eight pixels sharing pixelcircuitry and compact layout. The 8-shared pixel architecture 800C canbe used as the pixel architecture for pixels in the pixel array 112 insome embodiments. The 8-shared pixel architecture 800C includes eightpixels, and though many such pixels are arranged in the array 112, forsimplicity only one 8-shared pixel architecture 800C is shown in greaterdetail.

The 8-shared pixel architecture 800C includes eight photodiodes PD1-PD8and their corresponding transfer gates TG1-TG8, a floating diffusionnode FD, reset transistor RST, voltage supply VDD, source followeramplifier SF_AMP, selector transistor SEL, output voltage node Vcol, andcurrent source Ibias. These elements can perform similar functions asdescribed above with respect to FIG. 8A. The 8-shared pixel architecture800C also includes a storage capacitor CS1-CS8 and corresponding timingcircuitry TS1-TS8 connected in series between each of transfer gatesTG1-TG8 and the floating diffusion FD. Sharing of the floating diffusionFD, source follower amplifier SF_AMP, row select transistor SEL, andreset transistor RST, between adjacent photodiodes PD1-PD8 assists inincreasing the fill factor of the pixel architecture. In someembodiments, photodiodes PD1-PD8 can include two photodiodes used todetect a red component of incoming light, four photodiodes used todetect a green component of incoming light, and two photodiodes used todetect a blue component of incoming light.

As described above, the timing circuitry TS1-TS8 can include high-speedlogic configured to control accumulation of one or more charge dumpsfrom a photodiode PD1-PD8 in the associated storage capacitor CS1-CS8and to cause the accumulated charge to flow to the floating diode FD forpower-efficient extension of the small pixel's full well capacity in theanalog domain. The accumulated charge can be read out from the storagefloating diode FD and connected one of the storage capacitors CS1-CS8through the source follower SF_AMP to the output voltage node Vcol.

FIG. 8D illustrates an embodiment of a 16-shared image sensor pixelarchitecture 800D having full well capacity extension capabilities. Someembodiments of the image sensor can be a solid-state image sensor, forexample a CMOS image sensor, which has sixteen pixels sharing pixelcircuitry and compact layout. The 16-shared pixel architecture 800D canbe used as the pixel architecture for pixels in the pixel array 112 insome embodiments. The 16-shared pixel architecture 800D includes sixteenpixels, and though many such pixels are arranged in the array 112, forsimplicity only one 16-shared pixel architecture 800D is shown ingreater detail.

The 16-shared pixel architecture 800D includes sixteen photodiodesPD1-PD16 and their corresponding transfer gates TG1-TG16, a floatingdiffusion node FD, reset transistor RST, voltage supply VDD, sourcefollower amplifier SF_AMP, selector transistor SEL, output voltage nodeVcol, and current source Ibias. These elements can perform similarfunctions as described above with respect to FIG. 8A. The 16-sharedpixel architecture 800D also includes a storage capacitor CS1-CS16 andcorresponding timing circuitry TS1-TS16 connected in series between eachof transfer gates TG1-TG16 and the floating diffusion FD. Sharing of thefloating diffusion FD, source follower amplifier SF_AMP, row selecttransistor SEL, and reset transistor RST, between adjacent photodiodesPD1-PD16 assists in increasing the fill factor of the pixelarchitecture. In some embodiments, photodiodes PD1-PD16 can include fourphotodiodes to detect a red component of incoming light, eightphotodiodes to detect a green component of incoming light, and fourphotodiodes to detect a blue component of incoming light.

As described above, the timing circuitry TS1-TS16 can include high-speedlogic configured to control accumulation of one or more charge dumpsfrom a photodiode PD1-PD16 in the associated storage capacitor CS1-CS16and to cause the accumulated charge to flow to the floating diode FD forpower-efficient extension of the small pixel's full well capacity in theanalog domain. The accumulated charge can be read out from the storagefloating diode FD and connected one of the storage capacitors CS1-CS16through the source follower SF_AMP to the output voltage node Vcol.

FIGS. 8A-8B illustrate various pixel circuitry architectures that can beused to extend the full well capacity of small pixels. In someembodiments, having even numbers of shared pixels and/or multiples offour shared pixels can provide for a more compact image sensor layout,pixel symmetry, uniformity of pixel structure, and noise suppression.These examples are provided to illustrate, and not to limit, the fullwell capacity extension capabilities of having a storage capacitor andtiming circuitry associated with each photodiode. In other embodiments,other numbers of shared pixels (e.g., 32) or other pixel architecturesincluding fewer or greater numbers of transistors per pixel (e.g., 3T,5T, etc.) can be used together with the storage capacitor and timingcircuitry to extend pixel full well capacity in the analog domain.

FIG. 9A illustrates an example timing scheme for turning pixelarchitecture components on and off to perform an embodiment of a kTCnoise compensation process using the pixel architecture of any of FIGS.8A-8D. FIG. 9B illustrates a graphical representation of charge flowbetween pixel architecture components during the timing scheme of FIG.9A.

The timing scheme 900 can be used to accomplish a pixel readout processhaving eight steps in accordance with the illustrated timing points @t1through @t8. At timing point @t1 the photo-detector PD, floatingdiffusion FD, and storage capacitor CS are reset by turning on thetransfer gate TG, timing circuitry TS, and reset transistor RST. Attiming point @t2 the reset transistor RST is turned off, the reset noiseis read from the floating diffusion FD and storage capacitor CS, andthis is followed by light exposure and charge integration in thephotodiode PD. At timing point @t3 during charge integration,non-saturated charges stay in the photodiode PD while any saturatedcharges overflow to the floating diode FD and storage capacitor CS viathe transfer gate TG and timing circuitry TS. At timing point @t4 afterthe integration time is complete the timing circuitry TS is shut off. Attiming point @t5, signal charges are split into the storage capacitorCS, followed by resetting the floating diffusion FD by turning on thereset transistor RST. At timing point @t6 the reset noise is readoutfrom the floating diffusion FD. At timing point @t7, photon-generatedcharges are transferred to the floating diffusion FD as the transfergate TG is switched on. At timing point @t8 the charge is read out withthe timing circuitry TS turned on. Multiple-dump of signal chargesoccurs by repeating timing points @t7 & @t8 a number of times, and allcharges are collected at the floating diffusion FD and storage capacitorCS.

Implementing Systems and Terminology

Implementations disclosed herein provide systems, methods and apparatusfor full well capacity extension. One skilled in the art will recognizethat these embodiments may be implemented in hardware, software,firmware, or any combination thereof.

In some embodiments, the circuits, processes, and systems discussedabove may be utilized in a wireless communication device. The wirelesscommunication device may be a kind of electronic device used towirelessly communicate with other electronic devices. Examples ofwireless communication devices include cellular telephones, smartphones, Personal Digital Assistants (PDAs), e-readers, gaming systems,music players, netbooks, wireless modems, laptop computers, tabletdevices, etc.

The wireless communication device may include one or more image sensors,two or more image signal processors, a memory including instructions ormodules for carrying out the processes discussed above. The device mayalso have data, a processor loading instructions and/or data frommemory, one or more communication interfaces, one or more input devices,one or more output devices for example a display device and a powersource/interface. The wireless communication device may additionallyinclude a transmitter and a receiver. The transmitter and receiver maybe jointly referred to as a transceiver. The transceiver may be coupledto one or more antennas for transmitting and/or receiving wirelesssignals.

The wireless communication device may wirelessly connect to anotherelectronic device (e.g., base station). A wireless communication devicemay alternatively be referred to as a mobile device, a mobile station, asubscriber station, a user equipment (UE), a remote station, an accessterminal, a mobile terminal, a terminal, a user terminal, a subscriberunit, etc. Examples of wireless communication devices include laptop ordesktop computers, cellular phones, smart phones, wireless modems,e-readers, tablet devices, gaming systems, etc. Wireless communicationdevices may operate in accordance with one or more industry standardsfor example the 3rd Generation Partnership Project (3GPP). Thus, thegeneral term “wireless communication device” may include wirelesscommunication devices described with varying nomenclatures according toindustry standards (e.g., access terminal, user equipment (UE), remoteterminal, etc.).

The functions described herein may be stored as one or more instructionson a processor-readable or computer-readable medium. The term“computer-readable medium” refers to any available medium that can beaccessed by a computer or processor. By way of example, and notlimitation, such a medium may comprise RAM, ROM, EEPROM, flash memory,CD-ROM or other optical disk storage, magnetic disk storage or othermagnetic storage devices, or any other medium that can be used to storedesired program code in the form of instructions or data structures andthat can be accessed by a computer. Disk and disc, as used herein,includes compact disc (CD), laser disc, optical disc, digital versatiledisc (DVD), floppy disk and Blu-ray® disc where disks usually reproducedata magnetically, while discs reproduce data optically with lasers. Itshould be noted that a computer-readable medium may be tangible andnon-transitory. The term “computer-program product” refers to acomputing device or processor in combination with code or instructions(e.g., a “program”) that may be executed, processed or computed by thecomputing device or processor. As used herein, the term “code” may referto software, instructions, code or data that is/are executable by acomputing device or processor.

Software or instructions may also be transmitted over a transmissionmedium. For example, if the software is transmitted from a web site,server, or other remote source using a coaxial cable, fiber optic cable,twisted pair, digital subscriber line (DSL), or wireless technologiesfor example infrared, radio, and microwave, then the coaxial cable,fiber optic cable, twisted pair, DSL, or wireless technologies forexample infrared, radio, and microwave are included in the definition oftransmission medium.

The methods disclosed herein comprise one or more steps or actions forachieving the described method. The method steps and/or actions may beinterchanged with one another without departing from the scope of theclaims. In other words, unless a specific order of steps or actions isrequired for proper operation of the method that is being described, theorder and/or use of specific steps and/or actions may be modifiedwithout departing from the scope of the claims.

It should be noted that the terms “couple,” “coupling,” “coupled” orother variations of the word couple as used herein may indicate eitheran indirect connection or a direct connection. For example, if a firstcomponent is “coupled” to a second component, the first component may beeither indirectly connected to the second component or directlyconnected to the second component. As used herein, the term “plurality”denotes two or more. For example, a plurality of components indicatestwo or more components.

The term “determining” encompasses a wide variety of actions and,therefore, “determining” can include calculating, computing, processing,deriving, investigating, looking up (e.g., looking up in a table, adatabase or another data structure), ascertaining and the like. Also,“determining” can include receiving (e.g., receiving information),accessing (e.g., accessing data in a memory) and the like. Also,“determining” can include resolving, selecting, choosing, establishingand the like.

The phrase “based on” does not mean “based only on,” unless expresslyspecified otherwise. In other words, the phrase “based on” describesboth “based only on” and “based at least on.”

In the foregoing description, specific details are given to provide athorough understanding of the examples. However, it will be understoodby one of ordinary skill in the art that the examples may be practicedwithout these specific details. For example, electricalcomponents/devices may be shown in block diagrams in order not toobscure the examples in unnecessary detail. In other instances, suchcomponents, other structures and techniques may be shown in detail tofurther explain the examples.

Headings are included herein for reference and to aid in locatingvarious sections. These headings are not intended to limit the scope ofthe concepts described with respect thereto. Such concepts may haveapplicability throughout the entire specification.

It is also noted that the examples may be described as a process, whichis depicted as a flowchart, a flow diagram, a finite state diagram, astructure diagram, or a block diagram. Although a flowchart may describethe operations as a sequential process, many of the operations can beperformed in parallel, or concurrently, and the process can be repeated.In addition, the order of the operations may be re-arranged. A processis terminated when its operations are completed. A process maycorrespond to a method, a function, a procedure, a subroutine, asubprogram, etc. When a process corresponds to a software function, itstermination corresponds to a return of the function to the callingfunction or the main function.

The previous description of the disclosed implementations is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these implementations will bereadily apparent to those skilled in the art, and the generic principlesdefined herein may be applied to other implementations without departingfrom the spirit or scope of the invention. Thus, the present inventionis not intended to be limited to the implementations shown herein but isto be accorded the widest scope consistent with the principles and novelfeatures disclosed herein.

What is claimed is:
 1. An imaging system comprising: a pixel arrayincluding a plurality of photosensors; a plurality of readout circuitseach for reading light integrated in one of a plurality of photosensorsubsets coupled thereto, each of the plurality of photosensor subsetsincluding at least two photosensors of the plurality of photosensors,each of the plurality of readout circuits comprising: a plurality ofstorage capacitors, each of the plurality of storage capacitors incommunication with one corresponding photosensor of a photosensor subsetand having capacitance for storage of an accumulated charge representinga plurality of charge dumps from the corresponding photosensor, each ofthe plurality of charge dumps comprising a charge representing the lightintegrated in the corresponding photosensor during an integrationperiod; a plurality of timing circuits, each one of the plurality oftiming circuits associated with a corresponding one of the plurality ofstorage capacitors, each one of the plurality of timing circuitsconfigured for controlling a number and timing of the plurality ofcharge dumps to the corresponding one of the plurality of storagecapacitors from the corresponding photosensor; a floating diffusion nodein communication with each of the plurality of storage capacitors suchthat each of the plurality of storage capacitors can transfer theaccumulated charge held therein through the floating diffusion node; asource follower amplifier for receiving the accumulated charge from thefloating diffusion node and converting the accumulated charge into anelectrical signal, the floating diffusion node and source followeramplifier shared among the photosensor subset; an analog to digitalconverter in communication with the source follower amplifier to receivethe electrical signal representing the accumulated charge from thesource follower amplifier and convert the electrical signal representingthe accumulated charge into a digital signal representing theaccumulated charge; a memory configured to store the digital signalrepresenting the accumulated charge; and a noise compensation moduleconfigured to at least, for each digital signal readout, perform a firstread of the pixel array, the first read representing a kTC noise valuedetermined, for each timing circuit of the plurality of timing circuitsof each readout circuit of the plurality of readout circuits, at leastpartly by turning the timing circuit on to connect the corresponding oneof the plurality of storage capacitors in series with the floatingdiffusion node of the readout circuit, and reading a reset noise levelof at least the corresponding one of the plurality of storagecapacitors, the timing circuit, and the floating diffusion node of thereadout circuit; perform a second read of the pixel array, the secondread including the digital signal representing the accumulated charge,the noise compensation module configured to use the first read to reduceeffects of kTC noise on the digital signal, and store the first read andthe second read in the memory.
 2. The imaging system of claim 1, whereinthe noise compensation module is configured, for each digital signalreadout, to reset a storage capacitor of the plurality of storagecapacitors and the corresponding photosensor before performing the firstread of the pixel array, the first read representing kTC noise of thestorage capacitor, an associated timing circuit, and an associatedfloating diffusion node.
 3. The imaging system of claim 2, wherein thenoise compensation module is configured to perform the second read ofthe pixel array after conversion of the electrical signal representingthe accumulated charge into the digital signal representing theaccumulated charge, the second read representing the captured imagedata.
 4. The imaging system of claim 3, wherein the noise compensationmodule is configured to use the first read as a baseline for the secondread to reduce effects of the kTC noise on the digital signal.
 5. Theimaging system of claim 3, wherein the noise compensation module isconfigured to subtract the first read from the second read to reduceeffects of the kTC noise on the digital signal.
 6. The imaging system ofclaim 5, wherein subtracting the first read from the second read yieldsa differential frame, wherein the noise compensation module is furtherconfigured to store the differential frame in the memory, to compute atleast one additional differential frame, and to digitally aggregate thedifferential frames.
 7. The imaging system of claim 1, wherein thememory comprises DRAM.
 8. The imaging system of claim 1, wherein each ofthe plurality of readout circuits further comprises a reset transistorconfigured to reset one or both of the floating diffusion node and oneof the plurality of storage capacitors to a predetermined charge levelprior to the plurality of charge dumps.
 9. The imaging system of claim1, wherein each of the plurality of timing circuits is connected inseries with the floating diffusion node and the corresponding one of theplurality of storage capacitors is connected in series with theassociated timing circuit.
 10. The imaging system of claim 1, whereinthe capacitance of a storage capacitor of the plurality of storagecapacitors stores eight or more charge dumps from the correspondingphotosensor.
 11. The imaging system of claim 1, wherein the capacitanceof a storage capacitor of the plurality of storage capacitors is greaterthan or equal to approximately 3.2 femtofarads.
 12. A method forreducing an effect of kTC noise on captured image data, the methodcomprising: performing a first read of a pixel array comprising aplurality of photosensors, for each readout circuit of a plurality ofreadout circuits coupled to a subset of at least two photosensors of theplurality of photosensors, each of the at least two photosensors havingan associated capacitor and timing circuit coupled between thephotosensor and a floating diffusion node shared by the subset, and foreach of the at least two photosensors of the subset coupled to thereadout circuit: resetting a photosensor of the at least twophotosensors in the subset coupled to the readout circuit, resetting thestorage capacitor coupled between the photosensor and the floatingdiffusion, reading a reset noise signal representing a kTC noise of thestorage capacitor and the floating diffusion node by at least turningthe timing circuit on to connect the storage capacitor in series withthe floating diffusion node, and reading a reset noise level of at leastthe storage capacitor, the timing circuit, and the floating diffusionnode; storing a digital representation of the reset noise signal in amemory; performing a second read of the pixel array by, for each readoutcircuit of the plurality of readout circuits, and for each of the atleast two photosensors of the subset coupled to the readout circuit:integrating light in the photosensor for a first integration period,using the timing circuit to perform a plurality of charge dumps from thephotosensor to the associated storage capacitor coupled between thephotosensor and the floating diffusion, each of the plurality of chargedumps comprising a charge representing the light integrated in thephotosensor during the first integration period, the plurality of chargedumps transferred into the storage capacitor to form an accumulatedcharge, and reading out the accumulated charge from the storagecapacitor through the floating diffusion to the memory; and using thefirst read to reduce the effect of the kTC noise of the storagecapacitors associated with the plurality of photosensors on the secondread.
 13. The method of claim 12, wherein using the first read to reducethe effect of the kTC noise of the storage capacitor on the second readcomprises using the first read as a baseline for the second read. 14.The method of claim 12, wherein using the first read to reduce theeffect of the kTC noise of the storage capacitor on the second readcomprises subtracting the first read from the second read.
 15. Themethod of claim 12, further comprising performing a third read of thepixel array by, for each readout circuit of the plurality of readoutcircuits, and for each of the at least two photosensors of the subsetcoupled to the readout circuit: resetting the photosensor; resetting thestorage capacitor coupled between the photosensor and the floatingdiffusion; integrating light in the photosensor for a second integrationperiod; using the timing circuit to perform a second plurality of chargedumps from the photosensor to the associated storage capacitor coupledbetween the photosensor and the floating diffusion, each of the secondplurality of charge dumps comprising a charge representing the lightintegrated in the photosensor during the second integration period, thesecond plurality of charge dumps transferred into the storage capacitorto form a second accumulated charge; and reading out the secondaccumulated charge from the storage capacitor through the floatingdiffusion to the memory.
 16. The method of claim 15, further comprisingusing the first read as a baseline for the third read.
 17. The method ofclaim 15, further comprising subtracting the first read from the thirdread.
 18. The method of claim 15, further comprising digitallyaggregating values of the second read and values of the third read toextend a full well capacity of the pixel array.
 19. The method of claim15, wherein the first integration period and the second integrationperiod are different.
 20. The method of claim 15, wherein the firstintegration period and the second integration period are the same. 21.The method of claim 12, wherein reading out the accumulated charge fromthe storage capacitor to the memory comprises receiving the accumulatedcharge at an analog to digital converter and converting the accumulatedcharge into a corresponding digital pixel value.
 22. The method of claim21, further comprising storing the corresponding digital pixel value inthe memory.
 23. An imaging apparatus comprising: photosensing means forcollecting a charge representing light integrated in the photosensingmeans; a plurality of readout means each coupled to a subset includingat least two of the photosensing means, each of the plurality of readoutmeans comprising: a plurality of storage means each associated with oneof the at least two photosensing means of the subset, each storage meanshaving capacitance for storage of an accumulated charge representing aplurality of charge dumps from the photosensing means coupled thereto,each of the plurality of charge dumps comprising a charge representingthe light integrated in the portion of the photosensing means, aplurality of timing control means for controlling a number and timing ofthe plurality of charge dumps from the photosensing means to theassociated storage means, each of the plurality of timing control meanscommunicating with one of the plurality of storage means, means forreading a kTC value of the plurality of storage means and the pluralityof timing control means when coupled in series, and means for convertingboth the kTC value and the accumulated charge of a storage means fromeach the storage means coupled to the readout means into electricsignals; and means for using a digital representation of the electricalsignal representing the kTC value to reduce the effects of the kTC ofthe storage means on a digital representation of the electrical signalrepresenting the accumulated charge.
 24. The apparatus of claim 23,further comprising means for transferring the plurality of charge dumpsfrom the portion of the photosensing means to the storage capacitor. 25.The apparatus of claim 23, further comprising means for storing thedigital representation of the electric signal representing the kTC valueand the digital representation of the electrical signal representing theaccumulated charge.
 26. The apparatus of claim 23, further comprisingmeans for digitizing the electric signal representing the kTC value andthe electric signal representing the accumulated charge.
 27. Theapparatus of claim 26, further comprising means for generating a finalcaptured image by subtracting the digitized representation of the kTCvalue from the digitized accumulated charge or using the digitized kTCvalue the digitized representation of the accumulated charge.
 28. Anon-transitory computer-readable storage medium storing instructionsthat, when executed, cause a processor to perform operations comprising:receiving, from each readout circuit of a plurality of readout circuitseach coupled to a photosensor subset including at least two photosensorsof a plurality of photosensors of a pixel array, both: a plurality ofkTC values each representing kTC noise of one storage capacitor of aplurality of storage capacitors and an associated floating diffusionnode connected in series with the one storage capacitor, each of theplurality of storage capacitors coupled between one of the plurality ofphotosensors in the photosensor subset and the associated floatingdiffusion node of a readout circuit, and a plurality of accumulatedcharge values, each of the plurality of accumulated charge values beinga digital representation of an accumulated charge in one of theplurality of storage capacitors, the accumulated charge controlled by anassociated timing circuit of a plurality of timing circuits, each of theplurality of timing circuits coupled between one of the plurality ofstorage capacitors and the floating diffusion node of the readoutcircuit and configured for controlling a timing and number of aplurality of charge dumps from an associated photosensor of theplurality of photosensors to the one of the plurality of storagecapacitors, each of the plurality of charge dumps comprising a chargerepresenting the light integrated in the associated photosensor; andusing the plurality of kTC values to reduce an effect of the kTC noiseof the plurality of storage capacitors on the plurality of accumulatedcharge values.
 29. The non-transitory computer-readable storage mediumof claim 28, the operations further comprising subtracting the each ofthe plurality of kTC values from a corresponding one of the plurality ofaccumulated charge values.
 30. The non-transitory computer-readablestorage medium of claim 28, the operations further comprising using theeach of the plurality of kTC values as a baseline for a correspondingone of the plurality of accumulated charge values.